Organs of the Human Digestive Tract
The human digestive tract is a long tube, running from the mouth to the anus, in which food is broken down and absorbed, and its main organs are the mouth, esophagus, stomach, small intestine, large intestine and rectum. In brief: food is chewed in the mouth, carried by the esophagus to the stomach, digested further in the stomach and small intestine, where nutrients are absorbed, and the remaining waste passes through the large intestine and rectum. The whole tract is about 8 to 9 meters long in an adult.
Understanding the order of the organs and their jobs is the basis of the study of digestion. This guide follows a piece of food along the tract, describes each organ, compares them in a table, works through the journey of a sandwich and corrects common mistakes.
What is the digestive system?
The digestive system has two parts. The alimentary canal (digestive tract) is the continuous tube through which food passes. The accessory organs, including the salivary glands, liver, gallbladder and pancreas, make or store digestive secretions but food does not pass through them. The accessory organs are covered in a separate article.
Digestion has several stages: ingestion (taking food in), digestion (breaking it down), absorption (moving nutrients into the blood), and egestion (removing undigested material). Large food molecules like starch, protein and fat must be broken into small soluble molecules to cross the wall of the gut.
What happens in the mouth?
Digestion begins in the mouth. Teeth cut and grind food into small pieces, which increases the surface area for enzymes. Adults normally have 32 teeth of four types: incisors for cutting, canines for tearing, and premolars and molars for grinding.
The salivary glands release saliva, which moistens food and contains the enzyme amylase. Amylase starts the breakdown of starch to maltose. The tongue forms the chewed food into a ball called a bolus and pushes it to the back of the mouth for swallowing.
What does the esophagus do?
The esophagus is a muscular tube about 25 centimeters long that connects the throat to the stomach. When a bolus is swallowed, rhythmic waves of muscle contraction called peristalsis push it down. This works even if a person is upside down. The flap of tissue called the epiglottis covers the windpipe during swallowing so that food does not enter the lungs. No digestion of food occurs in the esophagus.
What does the stomach do?
The stomach is a muscular, J-shaped bag that can hold about 1 to 1.5 liters of food when it is full. Its walls churn the food, a form of mechanical digestion, and mix it with gastric juice.
- Hydrochloric acid gives the stomach a pH of about 1.5 to 2. It kills many bacteria and provides the right conditions for protease enzymes.
- Pepsin is a protease that begins the breakdown of proteins into shorter chains.
- Mucus coats the stomach lining and protects it from acid and enzymes.
The food becomes a thick liquid called chyme. Rings of muscle called sphincters at each end of the stomach control its entry and exit, and chyme is released into the small intestine in small amounts.
What does the small intestine do?
The small intestine is a coiled tube about 6 to 7 meters long, and it is the main site of digestion and absorption. It has three regions: the duodenum, the jejunum and the ileum. In the duodenum, enzymes from the pancreas and bile from the liver mix with the chyme. The intestinal wall also makes enzymes.
The inner surface has many finger-like projections called villi, and their cells carry even smaller microvilli. These greatly increase the surface area for absorption, and the total surface is often compared to the size of a tennis court. The villi contain capillaries and a lymph vessel that carry the absorbed nutrients away. This process is explained in more detail in a later article.
What do the large intestine and rectum do?
The large intestine is about 1.5 meters long and wider than the small intestine. It consists of the cecum, the colon and the rectum. Its main job is to absorb water and some minerals from the undigested material. Bacteria living in it, the gut microbiota, ferment some fiber and make certain vitamins, including vitamin K.
The remaining material becomes feces, which consist of undigested food, fiber, dead cells, bacteria and water. The rectum stores feces until they are removed through the anus. The release is controlled by two sphincters, one of which is under voluntary control. The appendix is a small pouch attached to the cecum, and its function is still debated.
Digestive organs compared
| Organ | Main function | Key secretion | Nutrient digested |
|---|---|---|---|
| Mouth | Chewing, mixing | Saliva (amylase) | Starch (start) |
| Esophagus | Transport by peristalsis | Mucus | None |
| Stomach | Churning, protein digestion | Acid, pepsin | Protein (start) |
| Small intestine | Digestion and absorption | Intestinal enzymes | All major types |
| Large intestine | Water absorption | Mucus | None |
| Rectum | Stores feces | Mucus | None |
Worked example: the journey of a sandwich
Follow a cheese sandwich along the tract. Bread provides starch, cheese provides protein and fat, and lettuce provides fiber.
- Mouth: The teeth cut the sandwich into small pieces. Amylase begins to turn starch into maltose, which is why bread tastes sweeter if chewed for a long time.
- Esophagus: The bolus is swallowed and pushed to the stomach by peristalsis in a few seconds.
- Stomach: Churning and acid break the bolus into chyme. Pepsin starts to split the cheese proteins. This stage lasts about two to four hours.
- Small intestine: Bile helps to emulsify the fat, and pancreatic and intestinal enzymes finish the digestion of starch, protein and fat. Glucose, amino acids, fatty acids and glycerol are absorbed.
- Large intestine: Water is absorbed from what remains. The fiber from the lettuce passes through largely unchanged, and some is fermented by bacteria.
- Rectum and anus: The feces are stored and then removed. The whole journey takes roughly one to three days.
The order mouth, esophagus, stomach, small intestine, large intestine, rectum is the one that most exam questions require. The timing varies between people and with the type of food.
How was digestion studied?
In the 1820s the US Army surgeon William Beaumont treated a patient, Alexis St. Martin, who had a gunshot wound that left an opening into his stomach. Beaumont observed and tested the digestion of different foods directly and showed that digestion is a chemical process involving gastric juice. In the early twentieth century, Ivan Pavlov studied the control of digestive secretions in dogs and received the Nobel Prize in Physiology or Medicine in 1904.
Which mistakes are common?
Mistake 1: Saying that most digestion happens in the stomach
The small intestine is the main site of chemical digestion and absorption. The stomach starts protein digestion.
Mistake 2: Thinking food passes through the liver and pancreas
Food travels only through the alimentary canal. The liver and pancreas send secretions into it.
Mistake 3: Believing that the large intestine absorbs most nutrients
It absorbs mainly water and salts. Nutrient absorption occurs largely in the small intestine.
Mistake 4: Confusing the esophagus with the windpipe
The esophagus carries food to the stomach, and the trachea (windpipe) carries air to the lungs.